Prior Art
Due to keen demands for lighter weight of automobile steel sheets and safety assurance at collisions of automobiles as main backgrounds, higher strength is required for steel sheets. However, workability is required even for the high strength steel sheets, and steel sheets capable of satisfying the requirements for both strength and workability are in keen demand. Heretofore, dual phase steel (which will be hereinafter referred to as "DP steel") comprising ferrite and martensite has been proposed for hot rolled steel sheets for use in the field that has required a good ductility. It is known that DP steel has a better strength-ductility balance than those of solid solution-intensified, high strength steel sheets and precipitation-intensified, high strength steel sheets, but its strength-ductility balance limit is at TS.times.T.E1.ltoreq.2,000. That is, DP steel fails to meet more strict requirements in the current situations.
As means capable of meeting the requirements in the current situations to attain TS.times.T.E1&gt;2,000, it has been proposed to utilize retained austenite. For example, Japanese Patent Application Kokai (Laid-open) No. 60-43425 discloses a process for producing a steel sheet containing retained austenite, which comprises hot rolling a steel sheet in a temperature range of Ar.sub.3 to Ar.sub.3 +50.degree. C., retaining the steel sheet in a temperature range of 450.degree. to 650.degree. C. for 4 to 20 seconds and coiling it at a temperature of not more than 350.degree. C., and also Japanese Patent Application Kokai (Laid-open) No. 60-165320 discloses a process for producing a steel sheet containing retained austenite, which comprises conducting high reduction rolling of a steel sheet at a finishing temperature of not less than 850.degree. C., at an entire draft of at least 80%, a total draft of at least 60% for final three passes and a draft of at least 20% for the ultimate pass, and then conducting cooling to 300.degree. C. or less at a cooling speed of at least 50.degree. C./s.
However, these conventional processes are not preferable in practice from the viewpoints of energy saving and productivity improvement, because of retention at 450.degree. to 650.degree. C. for 4 to 20 seconds during the cooling, coiling at a low temperature such as 350.degree. C. or less, high reduction rolling, etc. Furthermore, the workability of the steel sheets produced by these processes is at TS.times.T.E1&lt;2,400, which would not always have fully satisfied the level required by users. That is, steel sheets having a higher TS.times.T.E1 (desirably more than 2,400) and a high productivity process for producing such steel sheets have still been in demand. On the other hand, in view of the actual formability, not only a good strength-ductility balance, but excellent uniform elongability (stretchability), enlargeability or hole expansibility (enlargeability into a flange shape), bendability, secondary workability, and toughness are also required. Furthermore, in the service field of these steel sheets, spot welding is more and more used, and thus an excellent spot weldability is also required. Still further, not only a higher tensile strength, but also a higher yield ratio (higher yield strength) is required from the viewpoint of strength assurance.
That is, the field of actual application can be considerably broadened by satisfying these requirements at the same time.
(Problems to be solved by the invention)
The present invention provides a hot rolled, high strength steel sheet having an excellent workability, containing retained austenite and being capable of attaining TX.times.T.E1.gtoreq.2,000, which is over the limit of the prior art, and also a process for producing the same. Furthermore, the present invention provides a hot rolled, high strength steel sheet having an excellent formability (strength-ductility balance, uniform elongability, enlargeability, bendability, secondary workability and toughness), a high yield ratio and an excellent spot weldability at the same time and also a process for producing the same.